When object temperature is measured by radiation thermometry using a two-dimensional thermal imaging device or a one-dimensional scanning thermometer, normally, the emissivity of an object or the distribution thereof is unknown and changes depending on measurement conditions and the surface state. Thus, information of accurate surface temperature or temperature distribution can not be obtained from radiance detected by the thermal imaging device or the scanning radiation thermometer.
Moreover, even if the material emissivity of each area to be measured is known, in the case where fine emissivity distribution is present, there is also a problem in that apparent emissivity is different from the known emissivity due to the limitations of the imaging characteristics of the thermal imaging device.
In addition, various attempts have been made to correct for unknown emissivity, but unfortunately, there is no method suitable for measurement responding to a fast-changing object temperature.
The following methods have been previously applied to measure the temperature of an object whose emissivity is unknown in a non-contact manner by a radiation thermometer or a thermal imaging device.
(1) A method of measuring the radiance distribution after heating an object to a known temperature using a heater, in order to find the emissivity distribution of an object to be measured.
(2) A method of detecting two polarizations of a light beam in a spot-type radiation temperature measurement, measuring the object reflectance ratio in the two polarizations, and correcting emissivity from the ratio, as an emissivity correction technique in FLA (see Patent Document 1).
(3) A method of superimposing reflected light from a blackbody auxiliary radiating source on the object, regulating the temperature of the auxiliary radiation source so that the sum of the thermal radiance from the object and the reflected radiance from the object becomes equal to thermal radiance from the auxiliary radiating source, and measuring the temperature of the auxiliary radiating source at that time using a contact-type thermometer to determine the object temperature from the measured temperature (see Non-Patent Document 1).
(4) A method for obtaining the true object temperature through an arithmetic operation from measured radiances of a high-emissivity portion and a low-emissivity portion using an infrared radiation thermometer or a thermography capable of simultaneously measuring two or more points before and after changing the environmental temperature by means of an environmental radiance temperature switching device, such as a thermal infrared source as an auxiliary heat source before which a shutter is attached (see Patent Document 2).
However, each of these related arts have problems in the following points.
The method of (1) requires an additional process of heating the object using the heater and, in addition, a means to detect the object temperature at this time.
In the method of (2), polarizing optical elements are required, and thus the application of this method to long-wavelength infrared light used in low temperature radiation thermometry results in high cost. In addition, this method is not suitable for surface distribution measurement.
In the method of (3), the auxiliary radiation source is required to be a blackbody, but it is difficult to obtain a satisfactory blackbody having a planer form. When the auxiliary radiation source is not a blackbody, correction is required, but sufficient accuracy is not obtained. In addition, it is required to measure both the auxiliary radiating source and the object to be measured, and the above method cannot be applied to a case where a fast-changing object temperature is measured. In addition, the system structure becomes complicated.
In the method of (4), it is required to measure the temperature while switching the environmental temperature in a step form. The above method is not therefore suitable for measuring a fast-changing object temperature. In addition, the system becomes complicated.